1. Field of the Invention
The present invention relates to a technique for generating extreme ultraviolet light used, for example, to form a circuit pattern on a semiconductor wafer.
2. Description of the Related Art
As a method for fabricating semiconductor integrated circuits adopted as the storage elements, information processing elements and the like for a variety of electrical equipment, such as personal computers, mobile phones and navigation systems, there has been known lithography whereby to irradiate light to a mask with a circuit pattern formed thereon so as to transfer the circuit pattern of the mask onto a photosensitive resin, namely, photoresist, on a semiconductor wafer.
Currently, the g-ray of a high-pressure mercury lamp having a wavelength of 436 nm, the i-ray having a wavelength of 365 nm, a KrF excimer laser having a wavelength of 248 nm, and an ArF excimer laser having a wavelength of 193 nm are mainly used for the wavelengths of the light used with the lithography. Longer wavelengths of light tend to lead to lower resolutions of circuit patterns on photoresists, thus making it impossible to achieve a higher level of integration of semiconductors, i.e., miniaturized circuit patterns, with the light of the aforesaid wavelengths. As a solution, there have been provided a laser-produced plasma (LPP) light source and a discharge-produced plasma (DPP) light source adapted to generate extreme ultraviolet light (hereinafter referred to as the EUV light, as appropriate), which has still shorter wavelengths (refer to Japanese Patent Application Laid-Open No. 2008-130230).
The EUV light has a characteristic of being absorbed by glass, thus making it impossible to make, for example, a change of the path of light by a glass optical system, such as a lens. For this reason, a Mo/Si multilayer film, the reflectance of which reaches the peak at a short wavelength, namely, a 13.5-nm wavelength, is employed as the optical system, i.e., the reflecting mirror, to change the path of light.
Therefore, both the laser-produced plasma light source and the discharge-produced plasma light source are configured to be capable of producing light having a wavelength suited for the characteristics of the optical system (the light having a wavelength of 13.5 nm that minimizes a reflection loss in the optical system). To be more specific, the laser-produced plasma light source is configured to irradiate a powerful laser, namely, a YAG laser, to tin (Sn) or tin (Sn) compound, which is a target material, thereby to produce plasma light exhibiting an intense emission peak in the vicinity of 13.5 nm. The discharge-produced plasma light source is configured to pass and discharge a high electrical current of a steady frequency between a pair of electrodes thereby to generate plasma light, which includes a light component having a wavelength of 13.5 nm, between the electrodes.
According to the light sources, however, the generation of the plasma produces debris, i.e., impurities, which interfere with the transfer of a circuit pattern onto a semiconductor wafer. More specifically, in the laser-produced plasma light source, debris is produced from a target material, namely, the tin, which is solid at ordinary temperature, as plasma is generated. In the discharge-produced plasma light source, debris is produced from the electrodes as plasma is produced, so that the debris adheres to an optical system, a mask or a semiconductor wafer, interfering with the transfer of a circuit pattern onto a semiconductor wafer.